Re: [Qemu-devel] [PATCH RESEND v1 1/2] i386: Add Intel Processor Trace feature support

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jan 17, 2018 at 10:32:56AM +0000, Kang, Luwei wrote:
> > > > > On Mon, Jan 15, 2018 at 12:04:55 -0200, Eduardo Habkost wrote:
> > > > > > CCing libvirt developers.
> > > > > ...
> > > > > > This case is slightly more problematic, however: the new feature
> > > > > > is actually migratable (under very controlled circumstances)
> > > > > > because of patch 2/2, but it is not migration-safe[1].  This
> > > > > > means libvirt shouldn't include it in "host-model" expansion
> > > > > > (which uses the query-cpu-model-expansion QMP command) until we
> > > > > > make the feature migration-safe.
> > > > > >
> > > > > > For QEMU, this means the feature shouldn't be returned by
> > > > > > "query-cpu-model-expansion type=static model=max" (but it can be
> > > > > > returned by "query-cpu-model-expansion type=full model=max").
> > > > > >
> > > > > > Jiri, it looks like libvirt uses type=full on
> > > > > > query-cpu-model-expansion on x86.  It needs to use
> > > > > > type=static[2], or it will have no way to find out if a feature
> > > > > > is migration-safe or not.
> > > > > ...
> > > > > > [2] It looks like libvirt uses type=full because it wants to get
> > > > > >     all QOM property aliases returned.  In this case, one
> > > > > >     solution for libvirt is to use:
> > > > > >
> > > > > >     static_expansion = query_cpu_model_expansion(type=static, model)
> > > > > >     all_props = query_cpu_model_expansion(type=full,
> > > > > > static_expansion)
> > > > >
> > > > > This is exactly what libvirt is doing (with model = "host") ever
> > > > > since query-cpu-model-expansion support was implemented for x86.
> > > >
> > > > Oh, now I see that the x86 code uses
> > > > QEMU_MONITOR_CPU_MODEL_EXPANSION_STATIC_FULL and not just QEMU_MONITOR_CPU_MODEL_EXPANSION_FULL.
> > Nice!
> > > >
> > >
> > > So, I need to add Intel PT feature in "X86CPUDefinition
> > > builtin_x86_defs[]" so that we can get this CPUID in specific CPU
> > > model not only "-cpu host". Is that right?
> > 
> > The problem is that you won't be able to add intel-pt to any CPU model unless the feature is made migration-safe (by not calling
> > kvm_arch_get_supported_cpuid() from cpu_x86_cpuid()).
> 
> Hi Eduardo,
>     Have some question need you help clear. What is
>     "migration-safe" feature mean? I find all the feature which
>     included in CPU model (builtin_x86_defs[]) will make
>     "xcc->migration_safe = true;" in
>     x86_cpu_cpudef_class_init(). If create a Skylake guest on
>     Skylake HW and live migrate this guest to another machine
>     with old HW(e.g Haswell). Can we think the new feature or
>     cpu model(Skylake guest) which only supported in Skylake HW
>     is safe?

I mean matching the requirements so we can say the feature is migration-safe,
that means exposing the same CPUID data to the guest on any host, if the
machine-type + command-line is the same.  The data on CPUID[7] is OK (it
changes according to the command-line options only), but the data exposed on
CPUID[14h] on this patch is not migration-safe (it changes depending on the
host it is running).


> 
> > 
> > What's missing here is to either: (a) make cpu_x86_cpuid() return host-independent data (it can be constant, or it can be
> > configurable on the command-line); or (b) add a mechanism to skip intel-pt from "query-cpu-model-expansion type=static".
> 
> ==> it can be constant:
>     I think it shouldn't be constant because Intel PT virtualization can only supported in Ice Lake hardware now. Intel PT cpuid info would be mask off on old platform.
> ==> or it can be configurable on the command-line:
>     Because of I didn't add this feature in any CPU model. We only can enable this feature by "-cpu Skylake-Server,+intel-pt" at present.

Note that I'm talking about CPUID[14h], not CPUID[7].  The CPUID[7] bits are
safe because they are set according to the CPU model + command-line options
only.  The bits on CPUID[14h] change depending on the host and are not
migration-safe.  CPUID[7].EBX[bit 25] is just the (already configurable) bit
that enables the migration-unsafe code for CPUID[14h].

> 
> What about add a new cpu model name "Icelake" and add PT in this. Is that would be migration safe?

It won't, because of the CPUID[14h] code that makes it unsafe to migrate
between hosts with different Intel-PT capabilities (i.e. different data on
CPUID[14h]).


> 
> Thanks,
> Luwei Kang
> 
> > Probably
> > (a) is easier to implement, and it also makes the feature more useful (by making it migration-safe).
> > 
> > >
> > > Intel PT is first supported in Intel Core M and 5th generation Intel
> > > Core processors that are based on the Intel micro-architecture code
> > > name Broadwell but Intel PT use EPT is first supported in Ice Lake.
> > > Intel PT virtualization depend on PT use EPT.  I will add Intel PT to
> > > "Broadwell" CPU model and later to make sure a "Broadwell" guest can
> > > use Intel PT if the host is Ice Lake.
> > 
> > The "if the host is Ice Lake" part is problematic.  On migration-safe CPU models (all of them except "max" and "host"), the data seen
> > on CPUID can't depend on the host at all.  It should depend only on the machine-type + command-line options.
> > 
> > --
> > Eduardo

-- 
Eduardo